Xcode5 DP3 Fix: lastActiveWorkspaceWindow method no longer exists

My previous pull request allowed the plugin to load, however it crashes whenever the plugin attempts to get the current editor area because -[IDEWorkspaceWindow lastActiveWorkspaceWindow] no longer exists.

This pull request uses -[IDEWorkspaceWindow lastActiveWorkspaceWindowController] instead, since the window was only needed to obtain the controller.  It seems like this method is also available in Xcode 4.6.2, but I wasn't able to verify if it is present in older versions.